To analyze the amount of oil that is contained in a particular soil at a particular depth in the proximity of a subterranean well requires extraction of a sample of the well material. Analysis of this material yields the percent of fluid and/or gas contained therein which is utilized to determine the type of fluid, such as oil, contained therein and the pressure thereof. However, it is important in order to obtain an accurate analysis to extract the core in as intact a condition as possible. Since the fluid and gas are contained in the core material at a pressure dependent upon the depth of the well, extraction of this core to an environment with a lower pressure results in the fluid expanding somewhat and the gas coming out of solution. In addition, the "mobile oil" contained in the core may also drain or "bleed" out of the core and be lost. Mobile oil is oil that passes through the core material and is a function of the permeability and porosity of the core itself and the volume of fluid contained therein.
One method for retaining mobile oil is sponge coring which is disclosed in U.S. Pat. No. 4,312,414, issued to the present Applicant. Sponge coring comprises disposing a high porosity sponge on the interior surface of the inner barrel of the well coring apparatus. The core is then forced into the inner barrel with the sponge disposed about the sides thereof. The oil and/or gas contained in the core then "bleeds" into the sponge thereby retaining an accurate profile of the oil along the longitudinal axis of the core.
There are a number of problems incurred during sponge coring to achieve accurate data. One of these problems is in having the surface of the sponge contacting the actual surface of the core with no contaminants disposed therein. During normal drilling operations, drilling mud, or a similar lubricant, is circulated around the coring bit. This drilling mud has a tendency to "cake" on the core which, when it is pushed up into the sponge in the inner barrel, can impede bleeding of the oil and/or gas to the sponge for retention therein. This results in a certain degree of inaccuracy. This problem is exacerbated by the high differential pressures that can result within a bore hole due to the formation pressure and the pressure of the drilling mud within the bore hole. Therefore it is necessary to minimize the build-up of this filter cake.
In view of the above described disadvantages with sponge coring, there exists a need for a sponge coring apparatus with reduced field filter cake build-up on the core to increase the accuracy of sponge analysis.